This invention relates to a method and apparatus for bending an elongate workpiece.
Metal tubes used in aircraft, e.g. in hydraulic systems, are of many different shapes and sizes. A prototype tube, which constitutes a tube standard, is defined during design of the aircraft using a computer-aided design (CAD) system and is bent to fit that definition. The bends are all of the same nominal radius. Alternatively, a prototype is bent to fit a mock-up of the aircraft. Data defining the prototype tube ("tube model description") is generated using a manually operated, five axis, position-sensing probe, such as the probe sold by Eaton-Leonard Inc. under the trademark VECTOR 1. The tube model description may define the prototype tube in intersection point format. In this format, the tube model description defines the tube end points and intersection points in an X-Y-Z Cartesian frame of reference. An intersection point is the point defined by the intersection of the axes of two straight sections, or legs, of the tube. The prototype tube is numbered, labeled and placed in storage.
Production tubes (tubes for use in manufacture of aircraft) are manufactured from straight lengths of tube using a numerically-controlled tube bender. The bender operates under control of a bend program, which is generated from the tube model description by modifying it to take account of springback and other systematic effects. The bend program is generally stored in L-R-A format, where L stands for length, R stands for rotation, and A stands for angle of bend. A production tube is measured after bending using mechanical gauges, which reflect the tube model description. If the bent tube does not conform to the desired configuration for that tube, the operator of the tube bender must modify the bend program. The sequence of bending and modifying is repeated until a bent tube is a satisfactory fit in the fixture. When a satisfactory fit is obtained, a batch of, e.g. 8 to 15, tubes is bent.
It is necessary to carry out this sequence of operations when each batch is bent because of the possibility of variations from batch to batch in tube material, tube diameter, wall thickness and spring-back. In addition, the set-up of the tube bender might change from batch to batch and this may necessitate a change in the bend program.
Previous attempts to facilitate the operation of checking and, if necessary, modifying the bend program by automating the comparison of a bent tube with the prototype tube have been unsuccessful because the prior methods have been too slow or too inaccurate or have not been technically feasible. For example, attempts have been made to measure the configuration of production tubes using the same type of position-sensing probe as is used to generate the tube model description. However, this method was found to be slow and inaccurate, and required a human operator to control the positioning of the probe. One form of the VECTOR 1 probe requires contact with the tube and carries with it the possibility that the tube will be distorted. A version of the VECTOR 1 probe employs an infrared shadow detector instead of a contact probe. Although the shadow detector does not touch the tube, and therefore there is no danger that the tube will be distorted, manual positioning of the probe is still required.
EOIS Corporation has announced a non-contact tube inspection system in which the tube is placed on a light table and is inspected using at least two video cameras that view the tube.